The LLPT Hypothesis and Water Anomalies

The perplexing behavior of water has led to one of the most significant theoretical debates in physical chemistry: the hypothesis of a Liquid-Liquid Phase Transition (LLPT). This theory posits that in the deeply supercooled region of water—a state below the normal freezing point but where crystallization has not yet occurred—liquid water can exist in two distinct forms: a low-density liquid (LDL) and a high-density liquid (HDL).¹² The origin of water’s numerous thermodynamic anomalies, such as the increase in heat capacity upon cooling, is attributed to the existence of a hypothesized liquid-liquid critical point (LLCP) that terminates a first-order phase transition line between LDL and HDL.¹²

However, indirect evidence for the hypothesis exists in the form of two experimentally observed amorphous solid phases: high-density amorphous ice (HDA) and low-density amorphous ice (LDA).¹² It is widely theorized that these amorphous phases are the glassy states of the hypothetical HDL and LDL, respectively.¹²

Amorphous Ice in Comets

• Comets form in extremely cold environments, such as the Kuiper Belt or Oort Cloud, where temperatures are low enough for water to freeze into amorphous ice rather than crystalline ice.
• LDA and HDA are two forms of amorphous ice that differ in density and structure:
  • LDA is formed at very low temperatures and pressures.
  • HDA can form when LDA is compressed at low temperatures.

Why This Matters

• These amorphous phases are thought to be glassy states of hypothetical low-density liquid (LDL) and high-density liquid (HDL) water — part of the liquid-liquid phase transition hypothesis in supercooled water.
• In comets, amorphous ice can trap gases like CO, CO₂, CH₄, and others. When the comet approaches the Sun, the ice transitions to crystalline form, releasing these gases and contributing to the comet’s coma and tail.

Scientific Implications

• Studying these ice phases in comets helps scientists understand:
  • The thermal history of the comet.
  • The formation conditions in the early solar system.
  • The behavior of water under extreme conditions — relevant for planetary science and astrobiology

Research Note – needs further investigation.